The present invention relates to a joint detection-decoding receiver, and in particular, to a joint detection-decoding receiver using a multistage parallel interference canceller for selectively estimating a partial codeword corresponding to a state transition (branch).
A DS-CDMA (Direct Sequence Code Division Multiple Access) is widely used as a multiple access scheme used in a wireless channel shared by multiple users. In accordance with a DS-CDMA system, different signature sequences or pseudo-noise (PN) codes are assigned to the multiple users. A DS-CDMA receiver comprises a bank of matched filters in order to obtain a user data from a received signal.
However, the DS-CDMA system has a disadvantage of a performance degradation due to a MAI (multiple Access Interference) by other user signals. In order to overcome the disadvantage, a method for removing the MAI through a MUD (Multi-User Detection) has been proposed. The MUD may reduce an effect of the MAI as well as improving a system capacity and a service quality. Considering an uplink of the DS-CDMA system, the receiver of a base station has an information on a spreading sequence of every user. Therefore, the receiver may remove the MAI using the information.
A JDD (Joint Detection-Decoding) receiver is proposed as a novel method for detecting the multi-user. The JDD receiver utilizes an information on a channel code to simultaneously carry out the MUD and a channel decoding.
FIG. 1 is a block diagram illustrating a multi-user detection tranceiving system of a DS-CDMA system.
Referring to FIG. 1, in accordance with the DS-CDMA system, a sequence of information bits bk of a user k is encoded into a sequence of channel symbols dk, and the sequence of channel symbols dk is transmitted via a multi-user channel. The sequence of channel symbols dk is received at a receiver as an output yk of a matched filter bank 30 through the matched filter bank 30. The output yk not only includes the MAI due to other users but also has a correlation over a frame of the channel code. Therefore, a multi-user detector 40 detects a symbol estimate {circumflex over (d)}k for the other users from the output yk of the matched filter bank, and a channel decoder 50 calculates an information estimate {circumflex over (b)}k of the sequence of information bits bk through a decoding. An accuracy of the symbol estimation according to a channel encoding is improved and an effect of the MAI is reduced by the joint detection-decoding. However, it is disadvantageous that a complexity of the JDD receiver is linearly increased proportional to a number of repetitions and compensations of the MUD and a frame size of the channel code.
A PIC (Parallel Interference Canceller) is generally used in the DS-CDMA system in order to effectively remove the MAI The PIC predicts the symbol for entire users simultaneously in parallel. A PIC receiver including attempts to remove the MAI through a signal processing of each stage for the entire users. The PIC receiver has a much simpler structure compared to a general linear MUD receiver.
FIG. 2 is a block diagram illustrating a parallel interference canceller of a DS-CDMA system.
As shown, each stage of the PIC comprises estimators 101 and 201, and cancellers 102 and 202. The estimators 101 and 201 estimates the symbols for the entire users and the MAI for the entire users is removed based on the estimation. A first stage of the PIC employs a bank of a single-user matched filter. An accuracy of the prediction increases according to a number of the stages, and a performance of the PIC is also improved.
For the user k, when a current parallel interference cancellation stage is assumed to be l, the estimators 101 and 201 computes a symbol estimate {circumflex over (d)}k(l|l−1) from the a posteriori signal yk(l−1) of the previous stage. The symbol estimate {circumflex over (d)}k(l|l−1) is treated as a priori information for a cancellation next stage. The cancellers 102 and 202 removes the MAI included in a user signal yk(l) using a correlation matrix R.
                                          y            k                    ⁡                      (            l            )                          =                                            y              k                        ⁡                          (              0              )                                -                                    ∑                              i                ≠                k                                      ⁢                                          ρ                ik                            ⁢                                                                    d                    ^                                    i                                (                                  l                  ⁢                                                                                l                      -                      1                                        )                                                                                                          [Equation 1]            
A final decision is established based on the signal yk(l) of a last stage. However, the estimators 101 and 201 of the conventional PIC carries out a hard-decision of the symbol estimate {circumflex over (d)}k(l|l−1) through the posteriori signal yk(l−1). When the estimators 101 and 201 make an improper decision, the interference removal becomes inaccurate, thereby reducing the accuracy of the interference removal is drastically reduced passing through each stage. Therefore, an accurate symbol estimation of the estimators 101 and 201 is very important, and the JDD receiver improves the accuracy by using the channel code at the symbol estimation stage.
In accordance with the conventional JDD receiver, when a convolution code (n, m, l) having a code rate of m/n and a memory length of l is used as the channel code, an information sequence bk=[bk(1), . . . , bk(N)]=[bk(1), . . . , bk(mN)] having a length of mN of a k-th user is encoded into a code word dk=[dk(1), . . . , dk(N+l)]=[dk(1), . . . , dk(n(N+l))] having a length of n(N+1), where N is a frame size of a channel code.
Since the codeword is transmitted through a multi-user channel and an error correction is possible by a redundant portion of the channel code, the JDD receiver improves the accuracy of the estimation stage. However, the entire codeword should be processed in the estimators 101 and 201 of each stage and the cancellers 102 and 202, the JDD receiver is disadvantageous in that the complexity and a computation load of the receiver are increased.